It is apparently (currently) impossible. The DRLs have to be deactivated by the Volvo computer interface, and the computer doesn't provide an option to disable them. I think one of the optionas was to burn the Xenons instead of the dedicated DRLs, but you can do that just by turning the light switch all the way on.

On the other hand, you could turn the light switch to the middle position which would disable the DRLs, but would turn on the parking lights. Of course, that's illegal in most states.